Ceramic paint compositions are generally known to those ordinarily skilled in the art of applying ceramic paints to automotive and architectural glazings. Such ceramic paints are used, for example, to form opaque borders around the peripheral marginal surfaces of glazings used as windshields, sidelites, and backlites of motor vehicles. Frequently such paints are applied in successive layers in order to increase the thickness of the ultimately fired ceramic paint coating; provide an outer, exposed layer which will not adhere to the forming surface of a press bending mold; provide multi-colored decorative surface treatments; etc.
Ceramic paints or pastes generally contain a mixture of metal oxides which together act as a coloring agent. The metal oxides are non-reactive with one another, as well as non-reactive with other compounds contained in the ceramic paint or the glass to which the ceramic paint is applied, at temperatures up to about 1,300.degree. F. The mixture of metal oxides may be adjusted so as to achieve a specific color for the ultimately produced fired ceramic paint. For example, the opaque peripheral marginal paint bands fired onto automotive glazings are generally black.
Additionally, the ceramic paints generally known in the art contain one or more low melting glass frits, which melt at temperatures below 1,300.degree. F. These low melting glass frits are the materials which ultimately fuse the ceramic paint together, and to the glass sheet, to insure that the ceramic paint remains affixed to the glass sheet after being cooled to room temperature.
A vehicle is also generally mixed with the metal oxides and glass frit, to allow the ceramic paint to be applied to the surface of the glass sheet by conventional paint application processes. Generally, organic vehicles, e.g., pine oil, mineral oils, low molecular weight petroleum fractions, and the like, are used to allow the ceramic paint to be applied to the glass surface by brushing, spreading, or screen printing.
When multiple layers of ceramic paint are to be applied to the surface of a glass sheet, the process for doing so is known to be time consuming and wasteful of energy. A previously applied layer of ceramic paint must be dried before a successive layer of ceramic paint may be deposited thereover. Generally this drying process is accomplished by heating the glass sheet having the previously applied layer of ceramic paint thereon. The thermal energy, which is generally supplied by infrared radiant sources or direct fired heaters, is effective to both heat the glass sheet and volatize the organic vehicle from the paint, causing it to dry. Thereafter, the glass sheet having the dried layer of ceramic paint thereon must be cooled before a successive layer of ceramic paint may be deposited thereover. This cooling process generally involves the use of air blowers to remove heat from both the ceramic paint and the glass sheet by convective heat transfer. The finally applied layer of ceramic paint is generally not subjected to the known heating-then-cooling process. Instead, the glass sheet having the layer or layers of dried ceramic paint and the outer, exposed layer of undried ceramic paint thereon is conveyed through a heating chamber where the layers of ceramic paint are devitrified and "fired" onto the surface of the glass sheet.
Clearly, the steps of heating-then-cooling the glass sheet between successive applications of ceramic paint, in order to dry the previously applied layer of ceramic paint, add significant costs and time to the manufacture of glass sheets having multiple layers of ceramic paint thereon.
By way of example, U.S. Pat. No. 4,770,685 to Boaz discloses a method for manufacturing a formed glass sheet with paint thereon, wherein a layer of ceramic paint having a non-stick characteristic is deposited over an initial layer of ceramic paint which does not possess such a quality. It is disclosed that the initial layer of ceramic paint is dried by heating the glass sheet before the second non-stick layer of ceramic paint is applied thereover. This procedure, of course, requires energy and time for heating-then-cooling the glass sheet between the applications of the first and second layers of ceramic paint.
The need to dry ceramic paint layers exists, of course, only when dealing with ceramic paints having liquid vehicles which must be volatilized to solidify the paint. In an effort to eliminate the heating-then-cooling process required for the application of successive ceramic paint layers, other more expensive ceramic paints have been developed. For example, U.S. Pat. No. 4,684,389 to Boaz discloses a UV-based ceramic paint composition. Such a paint composition applied to the surface of a glass sheet may be "cured" by ultraviolet radiation prior to the application of a second layer of ceramic paint. It is also known to apply a hot-melt ceramic paint to the surface of a glass sheet. Such a paint "freezes" to the surface of the glass sheet, allowing the immediate application of a successive layer of ceramic paint thereover. Thus, efforts to eliminate the heating-then-cooling steps required for the application of successive layers of ceramic paints to a glass sheet have been directed toward eliminating the use of a liquid volatile vehicle in the ceramic paint compositions. Despite the knowledge of methods for quickly heating liquid materials, a process for quickly heating ceramic paint compositions containing volatile liquid vehicles, which process does not require a subsequent cooling step to cool the hot glass sheet, has not heretofore been discovered.
U.S. Pat. Nos. 3,472,200 to Gerling and 4,765,773 to Hopkins disclose apparatus and a method for applying and quickly drying highway-marking paint using microwave energy, so that motor vehicles may drive over the painted surface immediately after it is formed. The patents do not suggest the application of multiple layers of paint, nor the use of such a process in conjunction with ceramic paint layers applied to glass sheets.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention, nor that the prior art when considered in combination suggests the present invention absent the teachings herein.
It would be desireable to develop a process for applying successive layers of ceramic paint to a surface of a glass sheet, wherein the time and energy required for drying the paint layers between successive applications thereof would be minimized.